In response to the increasing seriousness of global environmental issues, energy issues, and the like, a solar cell is attracting attention as clean energy-generating means with no concern over depletion. In a case in which a solar cell is used outdoors such as on the roof of a building, it is usual to use the solar cell in a solar cell module form.
Generally, the solar cell module is manufactured in the following order. First, a crystalline solar cell element (hereinafter also referred to as a power generation element or a cell) formed of polycrystalline silicon or monocrystalline silicon, or a thin film-type solar cell element obtained by forming an extremely thin (several micrometers) film of amorphous silicon or crystal silicon on a glass substrate or the like is manufactured.
Next, to obtain a crystalline solar cell module, a protective sheet (a transparent surface protective member) for the solar cell module, an encapsulating material for solar cell, the crystalline solar cell element, an encapsulating material for solar cell, and a protective sheet (a back surface protective member) for the solar cell module are sequentially laminated.
Meanwhile, to obtain a thin film-based solar cell module, a thin film-type solar cell element, an encapsulating material for solar cell, and a protective sheet (a back surface protective member) for the solar cell module are sequentially laminated. After that, the solar cell module is manufactured using a lamination method or the like in which the above-described laminate is suctioned in a vacuum, heated and pressed. The solar cell module manufactured in the above-described manner is weather resistant and is also suitable for outdoor use such as on the roof of a building.
An ethylene/vinyl acetate (EVA) copolymer film is widely used as an encapsulating material for solar cell due to its excellent transparency, flexibility, adhesiveness and the like. However, in a case in which an EVA composition was used as a constituent material for an encapsulating material for solar cell, there was a concern over the possibility of a component being generated by the decomposition of EVA such as acetic acid gas affecting a solar cell element.
In contrast to the ethylene/vinyl acetate copolymer film, Patent Document 1 Japanese Unexamined Patent Publication No. 2010-258439) describes a resin composition for an encapsulating material for solar cell containing an ethylene/α-olefin copolymer satisfying specific conditions such as a density, a molecular weight, a melt viscosity, and the like.
In addition, in accordance with the recent distribution of solar power generation, the size of a power generation system has been increasing such as a mega solar power generation system, and also there is a movement of increasing the system voltage to decrease the transmission loss. In a solar cell module, an increase in the system voltage increases the potential difference between a frame and a cell. That is, generally, when the frame of the solar cell module is grounded, and the system voltage of a solar cell array reaches a voltage in a range of 600 V to 1000 V, the potential difference between the frame and the cell serves as the system voltage (in a range of 600 V to 1000 V) in a module in which the voltage becomes highest, and daytime power generation is maintained in a state in which a high voltage is applied. In addition, glass has a lower electric resistance compared with an encapsulating material, and a high voltage is generated between a glass plate and the cell through the frame. That is, in a state of daytime power generation, in modules connected in series, the potential difference between the cell and the module and the potential difference between the cell and the surface of a glass plate increase sequentially from the ground side, and the potential difference on the high voltage side of the system voltage is rarely changed in a portion with the largest potential difference. As a solar cell module used in the above-described state, there is a report regarding an example of a module in which a crystalline power generation element is used in which a potential induced degradation (PID) phenomenon in which the output is significantly decreased and the characteristics deteriorate occurs. As a result, there has been a demand for a higher volume resistivity for an encapsulating material for solar cell in direct contact with a solar cell element to solve the above-described problem.